1. Field of the Invention
This invention relates to space situational awareness and more particularly to the detection of objections in geosynchronous orbit about the Earth.
2. Description of the Related Art
An aspect of space situational awareness is the detection, cataloging and tracking of non-stellar objects in geosynchronous orbits. A geosynchronous orbit is an orbit around the equatorial plane (zero inclination plus or minus 15 degrees) of the Earth with the same rotation and same orbital period. A geosynchronous satellite flies over the same points on Earth at the same times each day. A circular orbit at zero inclination is referred to as geostationary. A geostationary satellite remains over the same point on Earth at all times. These satellites orbit at approximately 36,000 km above the Earth with an orbital period of 24 hours. Geosynchronous satellites are considered to be strategic assets by virtue of their enormous communication or intelligence gathering capabilities. Other man made objects such as final booster stages also remain in geosynchronous orbit.
The effectiveness of any geosynchronous surveillance system may be judged on several factors. Can the system collect data to generate a complete catalog of the geobelt in a timely fashion? How quickly and often can the system make a sweep of the entire geobelt to update the catalog? Can the system detect small objects (e.g. cross-section <0.1 m2)? Can the system resolve closely-spaced objects (CSOs) (e.g. <1 km)? Can the system be command to observe a specific portion of the geobelt? Can the system be fielded at relatively low cost?
The current state of the art is a land based optical system of telescopes at three ground sites around the globe known as “GEODSS”. The telescopes can only update the catalog at night and must image the objects from 36,000 km away through Earth's atmosphere. This limits the system's capability to detect both small objects and closely spaced objects.
SBSS-LEO injects an observer satellite into a low earth orbit (LEO) at 98 degrees of inclination, close to a polar orbit. The observer satellite orbits the Earth every 90 minutes at 160 to 2,000 km above the surface of the Earth. A telescope is mounted on a two-axis gimbal to scan the geobelt for objects at a range of approximately 36,000 km. A list of objects and their attributes (e.g. time, position and brightness) are transmitted from the observer satellite back to Earth where they are cataloged and tracked. SBSS-LEO uses the 90 minute orbit to perform a sweep of the geobelt and requires at least that long to detect a majority of the objects. SBSS-LEO's ability to resolve small and CSOs is limited by the fixed 36,000 km observation range. SBSS-LEO can be commanded to look in a specific direction but the Earth blocks one-half of the telescope's field-of-view (FOV) at all times. The aperture size required to resolve small objects and CSOs at approximately 36,000 km and the gimbal increase the weight of the satellite, which in turn dramatically effects cost.
GeoST-LEO injects an observer satellite having a body-fixed sensor in a zero inclination equatorial low earth orbit at approximately 160 to 2,000 km above the Earth with the sensor canted at a fixed angle from vertical. The satellite rotates once per 90 minute orbit to hold the angle. The frame transfer rate of the sensor is synchronized to the slow rotation of the satellite to perform time delay integration (TDI) to increase the integration period without smearing caused by motion of objects across the sensor's FOV. TDI of imaging sensors such as charge-coupled devices (CCDs) is widely used in other fields for observation of high speed moving objects. See Gibson et al. “Time-delay integration CCD read-out technique: image deformation” Mon. Not. R. astr. Soc. (1992) 258, 541-551; “Characteristics and use of back-thinned TDI-CCD” Hamamatsu, April 2007 and European Patent Application EP 1667428 entitled “Non-CCD imaging device with Tine Delay Integration (TDI) mode” published Jun. 7, 2006, which are hereby incorporated by reference. A list of objects and their attributes (e.g. time, position and brightness) are transmitted from the observer satellite back to Earth where they are cataloged and tracked. GeoST-LEO uses the 90 minute orbit to perform a sweep of the geobelt and, again, requires at least that long to detect substantially all of the objects. GeoST-LEO's ability to resolve small objects and CSOs is also limited by the fixed (approximate) 36,000 km observation range. GeoST-LEO can be commanded to look in a specific direction but again the Earth blocks one-half of the telescope's field-of-regard (FOR) at all times. The aperture size required to resolve small objects and CSOs at 36,000 km increases the weight of the satellite, which in turn dramatically effects cost.